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https://issues.apache.org/jira/browse/HADOOP-18321?focusedWorklogId=786901&page=com.atlassian.jira.plugin.system.issuetabpanels:worklog-tabpanel#worklog-786901
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ASF GitHub Bot logged work on HADOOP-18321:
-------------------------------------------

                Author: ASF GitHub Bot
            Created on: 01/Jul/22 01:08
            Start Date: 01/Jul/22 01:08
    Worklog Time Spent: 10m 
      Work Description: hadoop-yetus commented on PR #4521:
URL: https://github.com/apache/hadoop/pull/4521#issuecomment-1171818052

   :confetti_ball: **+1 overall**
   
   
   
   
   
   
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Issue Time Tracking
-------------------

    Worklog Id:     (was: 786901)
    Time Spent: 0.5h  (was: 20m)

> Fix when to read an additional record from a BZip2 text file split
> ------------------------------------------------------------------
>
>                 Key: HADOOP-18321
>                 URL: https://issues.apache.org/jira/browse/HADOOP-18321
>             Project: Hadoop Common
>          Issue Type: Bug
>    Affects Versions: 3.3.3
>            Reporter: Ashutosh Gupta
>            Assignee: Ashutosh Gupta
>            Priority: Critical
>              Labels: pull-request-available
>          Time Spent: 0.5h
>  Remaining Estimate: 0h
>
> Fix data correctness issue with TextInputFormat that can occur when reading 
> BZip2 compressed text files. When triggered this bug would cause a split to 
> return the first record of the succeeding split that reads the next BZip2 
> block, thereby duplicating that record.
> *When the bug is triggered?*
> The condition for the bug to occur requires that the flag 
> "needAdditionalRecord" in CompressedSplitLineReader to be set to true by 
> #fillBuffer at an inappropriate time: when we haven't read the remaining 
> bytes of split. This can happen when the inDelimiter parameter is true while 
> #fillBuffer is invoked while reading the next line. The inDelimiter parameter 
> is true when either 1) the last byte of the buffer is a CR character ('\r') 
> if using the default delimiters, or 2) the last bytes of the buffer are a 
> common prefix of the delimiter if using a custom delimiter.
> This can occur in various edge cases, illustrated by five unit tests added in 
> this change -- specifically the five that would fail without the fix are as 
> listed below:
>  # 
> BaseTestLineRecordReaderBZip2.customDelimiter_lastRecordDelimiterStartsAtNextBlockStart
>  # BaseTestLineRecordReaderBZip2.firstBlockEndsWithLF_secondBlockStartsWithCR
>  # BaseTestLineRecordReaderBZip2.delimitedByCRSpanningThreeBlocks
>  # BaseTestLineRecordReaderBZip2.usingCRDelimiterWithSmallestBufferSize
>  # 
> BaseTestLineRecordReaderBZip2.customDelimiter_lastThreeBytesInBlockAreDelimiter
> For background, the purpose of "needAdditionalRecord" field in 
> CompressedSplitLineReader is to indicate to LineRecordReader via the 
> #needAdditionalRecordAfterSplit method that an extra record lying beyond the 
> split range should be included in the split. This complication arises due to 
> a problem when splitting text files. When a split starts at a position 
> greater than zero, we do not know whether the first line belongs to the last 
> record in the prior split or is a new record. The solution done in Hadoop is 
> to make splits that start at position greater than zero to always discard the 
> first line and then have the prior split decide whether it should include the 
> first line of the next split or not (as part of the last record or as a new 
> record). This works well even in the case of a single line spanning multiple 
> splits.
> *What is the fix?*
> The fix is to prevent ever setting "needAdditionalRecord" if the bytes filled 
> to the buffer are not the bytes immediately outside the range of the split.
> When reading compressed data, CompressedSplitLineReader requires/assumes that 
> the stream's #read method never returns bytes from more than one compression 
> block at a time. This ensures that #fillBuffer gets invoked to read the first 
> byte of the next block. This next block may or may not be part of the split 
> we are reading. If we detect that the last bytes of the prior block maybe 
> part of a delimiter, then we may decide that we should read an additional 
> record, but we should only do that when this next block is not part of our 
> split *and* we aren't filling the buffer again beyond our split range. This 
> is because we are only concerned whether the we need to read the very first 
> line of the next split as a separate record. If it going to be part of the 
> last record, then we don't need to read an extra record, or in the special 
> case of CR + LF (i.e. "\r\n"), if the LF is the first byte of the next split, 
> it will be treated as an empty line, thus we don't need to include an extra 
> record into the mix.
> Thus, to emphasize. It is when we read the first bytes outside our split 
> range that matters. But the current logic doesn't take that into account in 
> CompressedSplitLineReader. This is in contrast to UncompressedSplitLineReader 
> which does.



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